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Emergence of low-energy electronic states in oxygen-controlled Mott insulator Ca$_{2}$RuO$_{4+δ}$

Insulator-to-metal transition in Ca$_{2}$RuO$_{4}$ has drawn keen attention because of its sensitivity to various stimulation and its potential controllability. Here, we report a direct observation of Fermi surface, which emerges upon introducing excess oxygen into an insulating Ca$_{2}$RuO$_{4}$, by using angle-resolved photoemission spectroscopy. Comparison between energy distribution curves shows that the Mott insulating gap is closed by eV-scale spectral-weight transfer with excess oxygen. Momentum-space mapping exhibits two square-shaped sheets of the Fermi surface. One is a hole-like $α$ sheet around the corner of a tetragonal Brillouin zone, and the other is an electron-like $β$ sheet around the $Γ$ point. The electron occupancies of the $α$ and $β$ bands are determined to be $n_α=1.6$ and $n_β=0.6$, respectively. Our result indicates that the insulator-to-metal transition occurs selectively in $d_{xz}$ and $d_{yz}$ bands and not yet in $d_{xy}$ band. This orbital selectivity is most likely explained in terms of the energy level of $d_{xy}$, which is deeper for Ca$_{2}$RuO$_{4+δ}$ than for Ca$_{1.8}$Sr$_{0.2}$RuO$_{4}$. Consequently, we found substantial differences from the Fermi surface of other ruthenates, shedding light on a unique role of excess oxygen among the metallization methods of Ca$_{2}$RuO$_{4}$.